“We take very seriously our constitutional duty to thoroughly and carefully vet each nominee sent to the Senate for confirmation,” said Senators Johnny Isakson of Georgia, the committee chairman, and Jon Tester of Montana, its top Democrat, in a joint statement. “We will continue looking into these serious allegations.”
Dr. Jackson, speaking with reporters on Capitol Hill before meeting with a Republican senator Tuesday afternoon, gave no indication that he would withdraw his nomination. He also did not answer questions about the accusations.
But he added, “I’m looking forward to getting it rescheduled and answering everybody’s questions.”
Members of Mr. Tester’s staff said that they had been given several credible accounts of Dr. Jackson being intoxicated during official White House travel. In several cases, they said, he had apparently grabbed his medical bag and was “attempting to assert himself,” to show he was in charge.
On one trip during Barack Obama’s presidency, White House staff needed to reach Dr. Jackson for medical reasons and found him passed out in his hotel room after a night of drinking, Tester aides said. The staff members took the medical supplies they were looking for without waking Dr. Jackson.
“He is the primary attendant of the president, the most powerful man in the world,” Mr. Tester said in an interview late Tuesday. “You don’t know when he is going to need you.”
Mr. Tester said that there was no evidence before the committee that Dr. Jackson had shown up drunk to the White House.
In response, White House officials described Dr. Jackson’s record as “impeccable,” and they distributed glowing comments that they said Mr. Obama wrote in Dr. Jackson’s annual military performance review.
“Ronny does a great job — genuine enthusiasm, poised under pressure, incredible work ethic and follow through,” Mr. Obama said of Dr. Jackson in 2016, according to information released Tuesday evening by White House officials.
Members of both parties also pushed back on the alcohol accusation. Senator Jerry Moran, Republican of Kansas, said that Dr. Jackson told him on Tuesday that “he has never had a drink while on duty.” Mr. Moran also said that Dr. Jackson did not specifically address other accusations against him.
Brian McKeon, who served as chief of staff for the Obama National Security Council, said he does not recall Dr. Jackson ever drinking to excess. “I am not even sure that I ever saw him in a hotel bar,” Mr. McKeon wrote in an email Tuesday.
Mr. Tester said that the committee had also received credible accusations that Dr. Jackson routinely distributed Ambien, a prescription sleep aid, which is not a narcotic, to White House staff and members of the news media flying on long overseas trips, as well as Provigil, a prescription drug for promoting wakefulness.
Mr. Tester said that he had spoken with John F. Kelly, the White House chief of staff, about the accusations on Monday. Mr. Kelly, he said, told him that the distribution of sleep aids and wakefulness drugs was “standard operating procedure.” Mr. Tester said he disagreed.
“We have a prescription drug problem in this country, and if we have doctors at the highest levels who are just handing them out like candy, we have a problem,” Mr. Tester said.
In a letter to the president on Tuesday, Mr. Tester and Mr. Isakson requested “any and all communication” between the Defense Department, the White House Military Office and the White House medical unit “regarding allegations or incidents” involving Dr. Jackson datingto 2006.
Mr. Tester said the committee had also received reports of a “toxic work environment” in the White House medical unit, which Dr. Jackson has overseen since 2013.
“That would involve belittling, screaming, verbally abusing the staff to the point where he would explode and the staff would feel they were on eggshells,” Mr. Tester said. He added that several of the people the committee had spoken with still work in the medical unit and are fearful of reprisals.
In 2012, a six-page report by the Navy’s medical inspector general found low morale and “unprofessional behaviors” as Dr. Jackson and his superior, Dr. Jeffrey Kuhlman, battled for control of the White House medical office.
Mr. Tester, a moderate Democrat up for re-election this fall, firmly rejected Mr. Trump’s assertion that the accusations had been politically motivated by partisan Democrats in Congress.
“We’ve had 12 appointees come before the committee, and I’ve supported every damn one of them,” he said. “I don’t want to play politics with the V.A.”
Republicans agreed the accusations are serious. Senator Thom Tillis, Republican of North Carolina and a member of the committee, said, “If the allegations were based in fact, it would be concerning.”
Dr. Jackson, a rear admiral in the Navy, was already expected to face difficult questioning during his testimony before the committee. Last month, Mr. Trump fired his first Veterans Affairs secretary, David J. Shulkin, an experienced hospital administrator and veteran of the department’s medical system, and then chose Dr. Jackson largely out of personal affinity.
As White House physician, Dr. Jackson had undergone intense vetting for a position that gives him unusually close access to the president. But Mr. Trump’s abrupt nomination of his personal doctor to lead the Department of Veterans Affairs pre-empted any review of Dr. Jackson’s qualifications to manage a large bureaucracy. Officials said there was virtually no examination of Dr. Jackson’s policy views before the announcement.
Before serving as a White House physician, Dr. Jackson had deployed as an emergency medicine physician to Taqaddum, Iraq, during the Iraq war.
Mr. Tester and Mr. Isakson said they would withhold a final judgment until they completed their investigation.
More than a half-dozen former White House officials who served with Dr. Jackson in Mr. Obama’s administration expressed support for him. None said they recalled him ever being drunk or loosely dispensing medications.
“He always seemed to be to be alert, responsive, responsible,” said David Axelrod, who served as Mr. Obama’s senior adviser. “My impressions were positive. My interactions were positive. I never heard any complaints.”
The turmoil around his nomination all but ensures that the Department of Veterans Affairs, the federal government’s second largest, will remain without a permanent leader for at least several weeks at a moment when it was supposed to be adopting systematic changes to its electronic health records system and to programs that allow veterans to seek care from private doctors at government expense.
The Senate received paperwork from the Trump administration formalizing Dr. Jackson’s nomination only last week.
“It has been really careless, maybe even negligent, about the vetting in a number of these nominations,” Senator Richard Blumenthal, Democrat of Connecticut, said of the White House.
Asked if he still supported the nominee, Senator Mitch McConnell, Republican of Kentucky and the majority leader, offered only, “We’re going to wait and see what Senator Isakson and the administration recommend.”
The White House defended Dr. Jackson’s record in a statement, but did not address the nature of the claims against him.
“Admiral Jackson has been on the front lines of deadly combat and saved the lives of many others in service to this country,” said Hogan Gidley, a deputy White House press secretary. “He’s served as the physician to three presidents — Republican and Democrat — and been praised by them all.”
Lawmakers were already preparing to press Dr. Jackson on his views on the role of private medical care for veterans, instead of the department’s government-run health care system. Senators also planned to challenge his lack of management experience running a large organization — the department employs more than 370,000 people and operates sprawling health and veterans benefits systems.
Before his nomination, Dr. Jackson had garnered little public attention. He took a rare turn in the spotlight in January, when he appeared on national television to announce the results of Mr. Trump’s first physical while in office. At the time, there was speculation over the president’s physical and mental health, and Dr. Jackson offered effusive compliments on both. Mr. Trump was pleased with the performance.
At one point, Dr. Jackson even quipped that given Mr. Trump’s genetics, he might live to 200 years old if he had a healthier diet.
